In town to promote The Bounty Hunter, the costars enjoyed a private river cruise on the Seine Saturday evening. Though the single stars have denied romance rumors, a source tells PEOPLE they looked as friendly and flirty as ever on their boat ride. "They were very close," the source says. "They really warmed up the room."

While a soft rain fell on the city, the stars and about 20 guests settled into wood-paneled grand salon on the top deck of the luxury charter yacht, ACTE III. Seated around a single long table, they dined on green salad and grilled chicken (for her) and lobster (for him). For dessert, they sipped champagne and enjoyed fresh berries.

Aniston arrived in Paris on Friday from Los Angeles, and dined that night at Ristorante La Stresa, a small exclusive eatery, where she bumped into Chinese actress Michelle Yeoh and French producer and director Luc Besson.

Butler arrived Saturday before the cruise, which took the stars past some of the City of Light's most famous landmarks, including the Place de la Concorde, the Seine bridges and the Statue of Liberty on the Ile des Cygnes, where the boat turned and headed toward the Eiffel Tower in time for its hourly lights display.

Aniston and Butler stayed close throughout the night, the source said, "hugging and touching each other" on the cruise, which docked later at the Quai d'Orsay.

With Sunday night scheduled for a Champs-Elysées red carpet premiere of their romantic comedy, the costars will then spend several days traveling across Europe.